Small quantities of contaminants are detrimental to the microchip fabrication process in the manufacturing of semiconductor electronic components. Contaminants may be introduced into the component from many sources such as residues from manufacturing process steps such as lithography, etching, stripping, and chemical mechanical planarization (CMP); particulates either indigenous to and/or resulting from manufacturing processes; inorganic particulates or materials such as native or chemical oxides, metal-containing compounds; or other sources. Contaminants, in the form of particulates, films, or molecules, can cause a variety of defects, such as short circuits, open circuits, and silicon crystal stacking faults. These defects can cause the failure of the finished component, such as microelectronic circuits, and these failures can cause significant yield reductions, which greatly increases manufacturing costs.
Microelectronic circuit fabrication requires many processing steps. Processing is performed under extremely clean conditions and the amount of contamination needed to cause fatal defects in microcircuits is extremely small. For example, an individual particle as small as 0.01 micrometer in size can result in a killer defect in a modern microcircuit. Microcontamination may occur at any time during the many steps needed to complete the microcircuit. Therefore, periodic cleaning of the components used for microelectronic circuits, such as wafers, is needed to maintain economical yields. Also, tight control of purity and cleanliness of the processing materials is required.
Cleaning is the most frequently repeated step in the manufacture of microelectronic circuits. At the 0.18-micrometer design rule, 80 of the approximately 400 total processing steps are cleaning steps. Wafers typically are cleaned after every contaminating process step and before each high temperature operation to ensure the quality of the circuit. Exemplary cleaning and removal applications include photoresist stripping/removal, particle/residue removal for post-chemical mechanical planarization (post-CMP cleaning), particle/residue removal for post-dielectric etching (or post-metal etching), and removal of metal contaminants.
Numerous cleaning methods have been used in the manufacture of semiconductor electronic components. These include immersion in liquid cleaning agents to remove contamination through dissolution and chemical reaction. Such immersion may also serve to reduce the van der Waals adhesive forces and introduce double layer repulsion forces, thereby promoting the release of insoluble particles from surfaces. A standard wet cleaning process in common use begins with exposure to a mixture of H2SO4, H2O2, and H2O at 110–130° C., and is followed by immersion in HF or dilute HF at 20–25° C. Next, a mixture of NH4OH, H2O2, and H2O at 60–80° C. removes particles and then a mixture of HCl, H2O2, and H2O at 60–80° C. removes metal contamination. Each of these steps is followed by a high purity H2O rinse. This wet cleaning process reaches fundamental barriers at dimensions less than 0.10 micrometer. As the device geometries shrink and gate oxide thickness decreases, sub-micrometer particle removal becomes increasingly difficult.
Stripping/removal of primarily organic photoresist may be performed using dilute aqueous mixtures containing H2SO4 and H2O2. Alternatively, the stripping/removal may be performed using a two-step plasma, or reactive ion etching (RIE) process, followed by wet chemical cleaning of the residue material. Ozonated H2O has been used for the decomposition of hydrocarbon surface contaminants on silicon wafers.
Brush scrubbing has been used to enhance the liquid immersion process by introducing hydrodynamic shear forces to the contaminated surfaces. A typical application uses a wafer cleaning apparatus comprising two opposed brushes for brushing a vertically disposed wafer in a tank that can contain a process liquid.
The addition of ultrasonic energy can increase the effectiveness of the liquid immersion process. Sound waves vibrating at frequencies greater than 20,000 cycles per second (20 KHz), i.e., beyond the range of human hearing, have been used to transmit high frequency energy into liquid cleaning solutions.
Wet processing methods may become problematic as microelectronic circuit dimensions decrease and as environmental restrictions increase. Among the limitations of wet processing are the progressive contamination of re-circulated liquids, re-deposition from contaminated chemicals, special disposal requirements, environmental damage, special safety procedures during handling, reduced effectiveness in deeply patterned surfaces due to surface tension effects and image collapse (topography sensitivity), dependence of cleaning effectiveness on surface wet-ability to prevent re-adhesion of contaminants, and possible liquid residue causing adhesion of remaining particles. Aqueous cleaning agents that depend upon chemical reaction with surface contaminants may also present compatibility problems with new thin film materials or with more corrosion-prone metals such as copper. Further, aqueous cleaning agents may introduce hydroxyl groups in porous low and ultralow dielectric constant materials, which may increase the dielectric constant of the material. In addition, the International Technology Roadmap for Semiconductors has recommended a 62% reduction in water use by the year 2005 and an 84% reduction by the year 2014 to prevent water shortages. With the continuing trend toward increasing wafer diameters having a larger precision surface area, larger volumes of liquid chemicals will be required in the fabrication process.
In view of these problems, methods for dry (anhydrous) surface cleaning of semiconductor electronic components are being developed. Among these is gas jet cleaning to remove relatively large particles from silicon wafers. However, gas jets can be ineffective for removing particles smaller than about 5 micrometers in diameter because the forces that hold particles on the surface are proportional to the particle size, while the aerodynamic drag forces generated by the flowing gas for removing the particles are proportional to the particle diameter squared. Therefore, the ratio of these forces tends to favor adhesion as the particle size shrinks. In addition, smaller particles are not exposed to strong drag forces in the jet since they normally lie within the surface boundary layer where the gas velocity is low.
Exposure to ozone combined with ultraviolet light can be used to decompose contaminating hydrocarbons from surfaces, but this technique has not been shown to remove inorganic contaminants or particles effectively.
Other alternatives to wet cleaning include the use of jets containing snow or pellet projectiles comprising frozen Ar, N2, H2O or CO2, which are used to “sandblast” contaminated surfaces. In these processes, pressurized gaseous or gas/liquid mixtures are expanded in a nozzle to a pressure near or below atmospheric pressure. The resulting Joule-Thomson cooling forms solid or liquid aerosol particles, which traverse the boundary layer and strike the contaminated surface. This technique requires extremely clean and pure processing materials. Trace molecular contaminants (e.g., hydrocarbons) in the feed gases can condense into solid particulates or droplets upon expansion, causing deposition of new contaminants on the surface. Although useful in providing removal of many surface contaminants, these processes cannot remove all of the important contaminants present on a wafer surface, and have not yet found wide acceptance in the semiconductor industry.
Immersion in supercritical fluids is another alternative to wet cleaning. The effectiveness of supercritical fluids in various cleaning and extraction applications is well established and extensively documented. The solvency of supercritical fluids is much greater than the corresponding gaseous state; thus, supercritical fluids can effectively dissolve and remove unwanted films and molecular contaminants from a precision surface. The contaminants can be separated from the cleaning agent by a reduction in pressure below the critical value, which concentrates the contaminants for disposal and permits recovery and re-use of the cleaning fluid.
Supercritical CO2 in particular has been used as a versatile and cost effective method to overcome the above-mentioned problems in wafer cleaning. Supercritical CO2 effectively cleans parts with increasingly smaller dimensions and lowers water usage, thereby yielding improvements in performance and environmental benefits. Preliminary Cost of Ownership (CoO) studies have shown that supercritical CO2 cleaning is also more cost effective when compared to aqueous cleaning. However, while liquid/supercritical CO2 by itself may be capable of dissolving primarily non-polar species, monomers and low molecular weight organic polymers, other species such as inorganic and/or polar compounds and high molecular weight polymers are not easily dissolved in either liquid or supercritical CO2. To remedy this lack of solvency, entrainers such as co-solvents and surfactants are added to the liquid or supercritical CO2 to increase contaminant solubility and thereby widen the range of contaminants that can be removed.
A wide variety of cosolvents, chelating agents and surfactants have been used/proposed for use with CO2 for semiconductor substrate cleaning. These include specific esters, ethers, alcohols, glycols, ketones, amines, amides, carbonates, carboxylic acids, alkane diols, alkanes, hydrogen peroxide, and chelating agents. Fluorinated and silicone-based surfactants have traditionally been used with liquid or supercritical CO2 for wafer cleaning applications because of their high solubility in CO2. These surfactants, however, are generally expensive and may increase overall processing costs.
Future microcircuits will have smaller feature sizes and greater complexities, and will require more processing steps in their fabrication. Contamination control in the process materials systems and processing environment will become even more critical. In view of these anticipated developments, there is a need for improved wafer cleaning methods to maintain or improve economical yields in the manufacture of these smaller and more complex microelectronic systems. In addition, the advent of smaller feature sizes and greater complexities will require improved fabrication processes steps including etching, thin film deposition, planarization, and photoresist development. Embodiments of the present invention, which are described below and defined by the following claims, address this need by improved processing methods utilizing dense cleaning fluids comprising lower cost, acetylenic alcohol or acetylenic diol processing agents and/or nitriles.